It is often advantageous to be able to drive more than one transmitting antenna, or to receive a signal from more than one receiving antenna. However, due to problems in impedance mismatch, it is not a simple matter of connecting more than one antenna to the respective input or output of a transceiver. Having more than one receive antenna, for instance, allows a degree of receive diversity to be employed and can increase the received signal strength.
Throughout the specification which follows, reference will be made to splitting or dividing a signal into two or more components, but the skilled person will appreciate that such description also includes combining two or more signals together, since both the prior art described and embodiments of the invention are intrinsically bi-directional.
Prior art techniques for splitting a signal from a single source to feed e.g. a pair of antennas can take a number of different forms. One particular technique uses the well-known Wilkinson Divider. This is shown in FIG. 1. It has the advantage of being relatively cheap, easy to design and implement and offers a predictable and relatively efficient performance at a given frequency. However, since the Wilkinson Divider relies on quarter-wavelength transformer elements, it is frequency dependent and so cannot offer good performance over anything other than a relatively narrow band. This can render it useless for certain wideband (or dual-band) applications.
The Wilkinson Divider of FIG. 1 has three ports labeled 1, 2 and 3. A signal applied to port 1 will be split and emerge as two identical signals from ports 2 and 3. The signal emerging from port 2 and 3 is attenuated by somewhat more than 3 dB compared to the signal input to port 1. In an ideal twin-output divider, the signal from each output port would be 3 dB down on the input signal. In a real Wilkinson Divider, the signal from each output is a little more than 3 dB down, due to losses in the balancing resistor.
Assuming that impedance of the transmitter applied to port 1 is 50 Ohm (Z0), then to ensure maximum power transfer to a pair of 50 Ohm loads, then the impedance at ports 2 and 3 needs to be the same. To ensure this, the path between ports 1 and 2 (and 1 and 3) needs to be a quarter wavelength at the frequency of operation. This sets the characteristic impedance of each branch to be Z0Λ/2=70.7 Ohm in this example. The Wilkinson divider requires the use of a balancing resistor between the two branches. This is set to a value of 2Z0=100 Ohm. The balance resistor increases the insertion loss of the device, but this is unavoidable in this device. It is desirable to realize the aim of splitting a signal or combining a plurality of signals in a simple manner, without the need for any discrete components, using only microstrip techniques.